The desire to integrate data, voice, image, video and other traffic over high speed digital trunks has led to the requirement for faster networks including the capability to route more information faster from one node to another node. A switch performs this routing of information. Generally, the switch consists of three logical elements: ports, a switch fabric and a scheduler.
Routing and buffering functions are two major functions performed by a switch fabric. New packets arriving at an ingress are transferred by the scheduler across the switch fabric to an egress. The ingress refers to a side of the switch which receives arriving packets (or incoming traffic). The egress refers to a side of the switch which sends the packets out from the switch.
Most of the switches today are implemented using a centralized crossbar approach. FIG. 1 is an exemplary illustration of a centralized crossbar switch. The packets arrive at the centralized crossbar switch 100 at multiple ingress ports 105 on the ingress 102. They are transferred across the switch fabric 110 to multiple egress ports 115 on the egress 104 and then sent out to an output link (not shown). The centralized crossbar switch 100 can transfer packets between multiple ingress port-to-egress port connections simultaneously.
A centralized scheduler controls the transfer of the packets from the ingress ports 105 to the egress ports 115. Every packet that arrives at the ingress ports 105 has to be registered in the centralized scheduler. Each packet then waits for a decision by the centralized scheduler directing it to be transferred through the switch fabric 110. With fixed size packets, all the transmissions through the switch fabric 110 are synchronized.
Each packet belongs to a flow, which carries data belonging to an application. A flow may have multiple packets. There may be multiple flows arriving at the ingress ports 105 at the same time. Since the packets in these multiple flows may be transferred to the same egress port, each of these packets waits for its turn in ingress buffers (not shown) in the ingress 102.
The centralized scheduler examines the packets in the ingress buffers and chooses a set of conflict-free connections among the appropriate ingress ports 105 and egress ports 115 based upon the configuration of the switch fabric 110. One of the egress ports 115 may receive packets from one or more ingress ports 105. However, at any one time, the centralized scheduler ensures that each ingress port is connected to at most one egress port, and that each egress port is connected to at most one ingress port.
Each packet transferred across the switch fabric 110 by the centralized scheduler waits in egress buffers (not shown) in the egress 104 to be selected by the centralized scheduler for transmission out of the switch. The centralized scheduler places the selected packets in the appropriate egress ports 115 to have the packets transmitted out to an output link.
There are different queuing disciplines used to select the packets from the egress queues. Fast queuing disciplines reduce overflow of the egress buffers and therefore prevent data loss. Traditionally, these queuing disciplines allow packets to be selected in serial. For example, a search for a next packet cannot be initiated until a packet is selected by a previous search. When a packet search and selection process takes “n” time slots, the output link has to wait for “n” time slots to receive a packet. When the search and selection process is complex to accommodate, for example, multiple traffic classes, the number “n” can be large, and it would take longer for the output link to receive a packet. Therefore, the serial approach to search and select packets is not efficient.